Author Topic: Can someone tell me what this does...(lock-in amplifier/voltmeter)  (Read 3780 times)

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Offline PStevenson

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can someone shed light on what this is/does maybe even how to use it. I've tried the net but any information regarding it usually assumes some prior knowledge. it was given to me to salvage parts from along with a load of other stuff however I was told this piece of equipment worked so I kept it + it looks cool in my "lab"

I have always been curious about it and I thought this might be the place to get some answers
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Offline tekfan

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Re: Can someone tell me what this does...(lock-in amplifier/voltmeter)
« Reply #1 on: July 10, 2011, 08:26:07 am »
Nice piece of test equipment you've got there. That and the Telequipment scope.
Are the inputs on the back? Can you take a shot of the back?

Here's what it does:
http://en.wikipedia.org/wiki/Lock-in_amplifier

Here's a pdf to describe it's use in practice (look at page 4 and onward):
http://www.physics.rutgers.edu/ugrad/387/nmr.pdf

And here's how the setup described in the pdf actually looks like:
http://www.physics.rutgers.edu/ugrad/387/388s06/photos/F05387/nmr.JPG
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Online ejeffrey

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Re: Can someone tell me what this does...(lock-in amplifier/voltmeter)
« Reply #2 on: July 10, 2011, 09:35:54 am »
Lockin amplifiers are the coolest instruments in the world :)  I have a few of that model sitting around, although most of them don't work any more.

A lock-in amplifier measures the amplitude and phase of a signal at a specific known frequency.  It multiplies the input signal by a sine wave at the known reference frequency.  Through the normal non-linear conversion process, this generates sum and difference frequency components.  For the special case of the component of the signal that is at the same frequency as the reference oscillator, the difference frequency is at 0 Hz or DC.  That signal is filtered with a low-pass filter, and produced as a slowly varying voltage or displayed on the meter.

The magic is that the analog multiplier followed by a low-pass filter is equivalent to a bandpass filter with the same bandwidth.  This is nice because it is easy to make a 0.01 Hz lowpass filter, but quite difficult to make a bandpass filter centered at 5 kHz, but with 0.01 Hz bandwidth.

The second important point is that the lock-in measurement is phase sensitive.  For any given frequency you can have a sine wave or a cosine wave, and they are distinct.  A single-phase lock-in measures only one, the component in phase with the reference signal (they will have an adjustment to change the reference phase to match the signal).  A dual phase lock-in measures both the sine and cosine terms independently.  This is how most LCR meters work: they inject an oscillating voltage and measure the conducted current with a dual-phase lockin.  The in-phase term gives the resistance, while the out-of-phase term gives the reactance.

Lockin measurements come in many guises, but in many ways they are a much more sophisticated cousin of a 'relative mode' measurement.  If you are trying to measure a small voltage with a DMM, you can compensate for the DC offset by in turn shorting the probes together and connecting them to the source you want to measure.  Subtracting the background reading from the signal, and you eliminate the offset error in your DMM -- as long as the offset doesn't drift in the time it takes you to switch measurements.   By hand you can only switch the signal at 0.25 Hz or so, but a lockin amplifier can work at 10 kHz.

A classic lockin use is to measure an optical signal in the presence of a huge background noise.  If you are trying to use a photodiode to measure the amount of light from a laser pointer scattered off a wall in broad daylight, you might expect it is impossible.  The sun is orders of magnitude brighter than the little laser pointer.  However, if you use an optical chopper to block and uncover the laser at a few kHz, a lock-in amplifier can recover the signal in the presence of much larger noise.
 

Offline PStevenson

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Re: Can someone tell me what this does...(lock-in amplifier/voltmeter)
« Reply #3 on: July 10, 2011, 08:04:08 pm »

that is pretty cool, I think I understand what it does - pin pointing a specific signal in a sea of noise to measure it basically.

the insides of it are brilliant though, it's got a great smell to it aswell so if you have some of this model and you haven't yet, take it apart !
thanks so much for the reply and explanation.

@Tekfan I can't really get it down off the shelf cause there's tons of stuff on it, that scope is a really nice one but unfortunately doesn't work properly, the trace has become really thick and is pretty much unusable, I keep getting it down and try to fix it but I have so far always failed.
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Online ejeffrey

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Re: Can someone tell me what this does...(lock-in amplifier/voltmeter)
« Reply #4 on: July 10, 2011, 08:58:49 pm »
Yeah, a lot of that old equipment is really beautiful inside.  I should see if I can take apart one of the PAR 124 lock-ins we have around and take some photos...
 

Offline PStevenson

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Re: Can someone tell me what this does...(lock-in amplifier/voltmeter)
« Reply #5 on: July 10, 2011, 10:11:36 pm »
you definitely should do, I'd love to see those pictures.
I usually take pictures of inside all the equipment I have for future viewing pleasure.
one of the most beautiful things I have for internals is a SE Labs EM102 scope, it's all discrete aswell
unfortunately that's broke too, it turns on and I can even get a good trace but it just drifts up the screen till it's gone
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